1. Field of the Invention
The present invention relates to a superconductive magnet utilized mainly in a magnetic resonance imaging (MRI) system, a nuclear magnetic resonance (NMR) analysis system, a monocrystal pulling system, and the like.
2. Description of the Related Art
In an MRI system, an NMR analysis system, and a monocrystal pulling system, because measurement or production of a monocrystal requires a high magnetic field, a superconductive magnet is utilized.
FIG. 12 is a cross-sectional perspective view illustrating the structure for a conventional superconductive magnet. FIG. 13 is a cross-sectional plan view of the conventional superconductive magnet in FIG. 12. A superconductive magnet is required to generate a high magnetic field in a desired region (referred to as a magnetic-field generation region 1, hereinafter). Taking a MRI superconductive magnet as an example, it is required to generate a high magnetic field of, for example, 1.5 tesla in a space (e.g., a spherical space having a diameter of 50 cm in a magnetic cylinder) for obtaining a diagnostic image. On the other hand, it is required that a magnetic field that leaks outside the superconductive magnet is as small as possible. For example, in the case of a MRI superconductive magnet, it is required that a leakage magnetic field at each position that is 5 m apart from the magnet is smaller than 0.5 millitesla. This is because it is desirable that a leakage magnetic field is as small as possible in a region where the leakage magnetic field attracts a magnetic body or affects an electronic apparatus or the like.
Accordingly, in order to reduce a leakage magnetic field, a conventional superconductive magnet incorporates two kinds of superconductive coils, i.e., a main coil 2 for generating a desired magnetic field and a shield coil 3 for cancelling out a magnetic field that leaks outside the main coil 2. The shield coil 3 is a coil for generating a magnetic field that is opposite the magnetic field generated by means of the main coil 2; by disposing the shield coil 3 at an appropriate position, a leakage magnetic field can be cancelled out.
Hereinafter, letting the center of the magnetic-field generation region 1 be an origin, the axis-direction coordinate and the radial coordinate are referred to as the Z coordinate and the R coordinate, respectively. The main coil 2 and the shield coil 3 are each formed of a plurality of superconductive coils that are arranged coaxially with one another. In order to secure the performance of the superconductive magnet while suppressing the cost as much as possible, the positions and the sizes of the main coil 2 and the shield coil 3 are selected in such a way that the specifications of required magnetic-field intensity, magnetic-field homogeneity, and leakage-magnetic-field intensity are satisfied and the amount of superconductive wires, which are main materials of the coils, is minimized.
As a result, in general, the main coils 2 are arranged in such a way as to adhere to the inner-circumference surface, of the cylinder, where the R coordinate is small and which is nearest to the magnetic-field generation region 1; the shield coils 3 are arranged in such a way as to adhere to positions (in FIG. 13, the four corners of the inner region of the magnet) where the R coordinate and the Z coordinate are maximum and which are furthest from the magnetic-field generation region 1. Because being a coil having a reverse polarity, the shield coil 3 has an effect to cancel out the magnetic field in the magnetic-field generation region 1, by arranging the shield coils 3 in such a way as to be far from the magnetic-field generation region 1, the magnetic field can more effectively be generated. The foregoing arrangement has the highest efficiency in terms of the amount of used superconductive wires; thus, the amount of superconductive wires can be reduced.
The main coil 2 and the shield coil 3 are each held in such a way as to be wound around a main bobbin 21 and a shield bobbin 31, respectively. The shield bobbin 31 is supported by the shield bobbin supporting member 32 in such a way as to be coupled with the main bobbin 21. In order to make the magnet generate a desired high magnetic field, it is required to apply a large current of several hundreds amperes to the superconductive coil. As a result, a high magnetic field is generated in the vicinity of each of the superconductive coils; therefore, large electromagnetic force is exerted on the superconductive coil. When the electromagnetic force makes the superconductive coil move or bend, heat is produced in the outer surface or the inside of the coil; this heat may cause a so-called quench phenomenon in which the superconductive state is destructed and transits to a normal conductive state. In a conventional superconductive magnet, in order to prevent the quench phenomenon, the main bobbin 21, the shield bobbin 31, and the shield bobbin supporting member 32 are formed of a thick material or formed in a rigidly coupled structure so that the electromagnetic force is suppressed from moving or bending the superconductive coil. Additionally, the shield bobbin 31 is formed of a cylinder integrated with the magnet approximately over the whole length thereof, in order to raise the rigidity of the shield bobbin 31. In addition, Japanese Patent Application Laid-Open No. 2007-288193 typifies the reference patent documents in the technical field to which the present invention belongs.
In recent years, the intensity of the magnetic field generated in a superconductive magnet has been raised, and it tends to be required that the system per se is compact. As a result, the intensity of the magnetic field in the vicinity of the superconductive coil has also been raised, and the electromagnetic force has also become large. In the conventional technology, by forming the main bobbin 21, the shield bobbin 31, and the shield bobbin supporting member 32 in a more rigid structure, a structure that can withstand electromagnetic force has been realized; however, there has been a problem that, the amount of materials utilized in the bobbins and the supporting structure increases as the electromagnetic force of a superconductive magnet is enlarged lately, and hence the weight and the costs in production, cooling, and transportation increase.